Barium fluoride high repetition rate UV excimer laser

ABSTRACT

The invention relates to an excimer laser which includes a source of a laser beam and one or more windows which include barium fluoride. Another aspect of the invention relates to an excimer laser which includes a source of a laser beam, one or more windows which include barium fluoride and a source for annealing the one or more windows. Another aspect of the invention relates to a method of producing a predetermined narrow width laser beam.

The present application claims priority to U.S. Provisional PatentApplication No. 60/273,028, filed Mar. 2, 2001, which is herebyincorporated by reference.

FIELD OF THE INVENTION

The subject invention is directed generally to an excimer laser; and inparticular to an excimer laser having a first or primary laser beam anda second or secondary laser beam suitable for annealing opticalcomponents.

BACKGROUND OF THE INVENTION

Throughout this application various publications are referenced. Thedisclosures of each of these publications in their entireties are herebyincorporated by reference in this application.

Excimer lasers have been conventionally developed for commercial use asa light source of a reducing projection and exposure device for asemiconductor manufacturing apparatus, because an excimer laser enablesextremely precise work.

Light oscillating from the excimer laser has various wavelengthcomponents and the central wavelength varies. As a result, if the lightis in the as-is status, an aberration will occur when the light passesthrough an external optical element, such as a lens, thereby reducingthe accuracy of the work. For this reason, there is a widely used art ofmaking a narrow band, in which an excimer laser is equipped with awavelength selecting element, such as a grating, to narrow the spectralwidth of the laser oscillation wavelength and to stabilize the centralwavelength as a central value of the oscillation wavelength.

U.S. Pat. No. 6,181,724 discloses an excimer laser. Laser gas is sealedin a laser chamber and energy is supplied as a result of an electricaldischarge in a discharge electrode, causing the laser beam to oscillate.The oscillating laser beam exits through a rear window, the beam size iswidened while passing through a first prism and a second prism, and thenthe laser beam enters a grating. In the grating, an angle relative tothe light path of the laser beam is controlled by an actuator and byoscillating the laser beam at a predetermined wavelength, a narrow bandis achieved. A group of optical components, which are the first prism,the second prism, and the grating, is collectively called thenarrow-band optics. The laser beam, with the wavelength being controlledby the narrow-band optics, passes through a front window and a frontmirror, which is a partial reflecting mirror, and part of the laser beamexits the laser chamber.

Typically, synthetic fused silica or calcium fluoride is used as thematerial of the optical components for the excimer lasers, however,there are significant disadvantages in using these materials. In orderto manufacture semiconductors efficiently in large quantities, there hasbeen a demand to increase the power of a laser by increasing the laseroscillation pulse numbers per unit time (also called the repetitionfrequency or repetition rate). However, the energy density is high inthe resonator of the laser, and moreover, a laser beam reciprocates inthe resonator and passes through the optical components many times. Forthis reason, as the power of a laser becomes higher, the opticalcomponents are deteriorated as a result of even minor distortion orunevenness inside the material. Even minor deterioration of the opticalcomponents exerts a great influence on the quality of the oscillatinglaser beam. Thus, the optical components of synthetic fused silica orcalcium fluoride are insufficient in durability when the power of anexcimer laser is increased, and a highly accurate control of thewavelength of an excimer laser is difficult when these opticalcomponents are used.

SUMMARY OF THE INVENTION

The present invention relates to an excimer laser which includes asource of a laser beam and one or more windows which include bariumfluoride.

Another aspect of the present invention includes an excimer laser whichincludes a source for a laser beam, one or more windows which includebarium fluoride, and a source for annealing the one or more windows.

Yet another aspect of the present invention includes an excimer laserwindow which includes barium fluoride.

Yet another aspect of the present invention includes a method ofproducing a predetermined narrow width laser beam. The method includesoscillating a laser beam whereby the laser beam exits a first window ofa chamber, widening the laser beam through one or more prisms,controlling the laser beam to a predetermined narrow width, and passingthe predetermined narrow width laser beam through a second window of thechamber, where the first and second windows of the chamber includebarium fluoride.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of this invention will beevident from the following detailed description of preferred embodimentswhen read in conjunction with the accompanying drawings in which:

FIG. 1 illustrates an excimer laser according to one embodiment of thepresent invention.

FIG. 2 illustrates an excimer laser according to an alternativeembodiment of the present invention.

FIG. 3 illustrates an excimer laser according to an alternativeembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an excimer laser which includes asource of a laser beam and one or more windows which include bariumfluoride.

Excimer laser devices are described in U.S. Pat. Nos. 6,181,724,6,282,221, 6,067,311 and 6,014,398, which are hereby incorporated byreference.

Typically, an excimer laser operates as follows. Laser gas is sealed ina laser chamber and energy is supplied to the gas by an electricaldischarge in a discharge electrode. This causes the laser beam tooscillate. The oscillating laser beam exits the laser chamber through arear window, the size of the laser beam is widened while passing throughprisms, and the laser beam enters a grating. In the grating, an anglerelative to the light path of the laser beam is controlled by anactuator and by oscillating a predetermined wavelength a narrow bandwidth is achieved. The laser beam, with the controlled wavelength passesthrough a front window and a front mirror and part of the laser beamexits the laser chamber.

With this general view of the operation of an excimer laser in mind,FIG. 1 illustrates one embodiment of an excimer laser of the presentinvention. As shown in FIG. 1, excimer laser device 10 includes aplurality of prisms such as first and second (and optionally third)prisms 12, first mirror 13, second mirror 15, a grating 14, and first 16and second 18 windows. First window 16 and second window 18 in laserchamber 17 form an ordinary Brewster angle relative to a laser beam 11in order to reduce energy loss. Components, such as plurality of prisms12, first mirror 13 and second mirror 15, and first window 16 and secondwindow 18 are made of a fluoride optical material, such as calciumfluoride, barium fluoride or magnesium fluoride. In one embodiment,these components are made of barium fluoride. In an alternativeembodiment, first window 16 and second window 18 are made of bariumfluoride and other components, such as plurality of prisms 12, firstmirror 13 and second mirror 15 are made of other materials, such ascalcium fluoride.

Another aspect of the present invention includes a method of producing apredetermined narrow width laser beam. The method includes oscillating alaser beam where the laser beam exits a first window of a chamber,widening the laser beam through one or more prisms, controlling thelaser beam to a predetermined narrow width, and passing thepredetermined narrow width laser beam through a second window of thechamber, where the first and second windows of the chamber includebarium fluoride.

In use, referring to FIG. 1, laser gas (such as Ar, Kr, Ne and/or F₂) issealed in laser chamber 17 and energy is supplied to the laser gas by anelectrical discharge in a discharge electrode (not shown). This causeslaser beam 11 to oscillate. Oscillating laser beam 11 exits the laserchamber 17 through a rear window 16. Laser beam 11 passes through prisms12, is reflected by second mirror 15 and grating 14. In second mirror15, an angle relative to the light path of laser beam 11 is controlledby an actuator 19. By oscillating laser beam 11 at predeterminedwavelength, a narrow band width is achieved for laser beam 11. Laserbeam 11 is totally reflected by grating 14 and second mirror 15 causinglaser beam 11 to reverse its original path and exit chamber 17 fromfront window 18 and exit from first mirror 13.

FIG. 2 is a second embodiment of the present invention. Excimer laserdevice 20 includes a plurality of prisms such as first and second prisms22, mirror 25, grating 24, and first 26 and second 28 windows. Firstwindow 26 and second window 28 in laser chamber 27 form an ordinaryBrewster angle relative to a laser beam 21 in order to reduce energyloss. In this embodiment, laser beam 21, is partially reflected bymirror 25, which is a partially reflecting mirror, and part of laserbeam 21 exits laser chamber 27. Components, such as plurality of prisms22, second mirror 25, and first window 26 and second window 28 are madeof a fluoride optical material, such as calcium fluoride, bariumfluoride or magnesium fluoride. In one embodiment, these components aremade of barium fluoride. In an alternative embodiment, first window 26and second window 28 are made of barium fluoride and other components,such as plurality of prisms 22 and mirror 25 are made of othermaterials, such as calcium fluoride.

Another aspect of the present invention includes an excimer laser windowmade of barium fluoride.

The laser windows of the present invention made of barium fluoridemaintain durability over a long operational life of the excimer laser.As used herein, “maintain durability” means that the barium fluoridewindows means that the magnesium fluoride windows have no perceptibleinduced absorption. The barium fluoride windows maintain durability fora laser having a output of greater than or equal to 10 mJ and arepetition rate of greater than or about 4 KHz. Further, the bariumfluoride windows of the present invention maintain durability for alaser having a output of greater than or equal to 10 mJ and a repetitionrate of greater than or about 4 KHz for over 500 million pulses and,optionally, for over 900 million pulses.

Another aspect of the present invention includes an excimer laser whichincludes a source for a laser beam, one or more windows which includebarium fluoride, and a source for annealing the one or more windows.

FIG. 3 shows an embodiment of the present invention which includes asource for annealing the windows of the laser chamber. As shown in FIG.3, excimer laser device 30 includes a plurality of prisms such as firstand second (and optionally third) prisms 32, first mirror 33, secondmirror 35, a grating 14, and first 16 and second 18 windows. Firstwindow 36 and second window 38 in laser chamber 37 form an ordinaryBrewster angle relative to a laser beam 31 in order to reduce energyloss. Components, such as plurality of prisms 32, first mirror 33 andsecond mirror 35, and first window 36 and second window 38 are made of afluoride optical material, such as calcium fluoride, barium fluoride ormagnesium fluoride. In one embodiment, these components are made ofmagnesium fluoride. In an alternative embodiment, first window 36 andsecond window 38 are made of magnesium fluoride and other components,such as plurality of prisms 32, first mirror 33 and second mirror 35 aremade of other materials, such as calcium fluoride.

Typically, a first laser beam (as described above) and a second laserbeam are generated by discharge electrode in laser chamber 37. In use,referring to FIG. 3, a second laser gas (such as Ar, Kr, Ne and/or F₂)is sealed in laser chamber 37 and energy is supplied to the second lasergas by a second electrical discharge in a second discharge electrode(not shown). This causes a second laser beam 31 to oscillate.Oscillating laser beam 31 exits the laser chamber 37 through a rearwindow 36. Laser beam 31 passes through prisms 32, is reflected bysecond mirror 35 and grating 34. In second mirror 35, an angle relativeto the light path of laser beam 31 is controlled by an actuator. Byoscillating laser beam 31 at predetermined wavelength, a narrow bandwidth is achieved for laser beam 31. Laser beam 31 is totally reflectedby grating 34 and second mirror 35 causing laser beam 31 to reverse itsoriginal path and exit chamber 37 from front window 38 and exit fromfirst mirror 33.

Second laser beam 31 is used to anneal first and second windows 38 and36 concurrently with operation of the excimer laser. The windows areirradiated with second laser beam 31 with light having a wavelength ofabout 250 nm. This wavelength corresponds to the wavelength of theinduced absorption band. Alternately, second laser beam 31 is usedeither before or after operation of the excimer laser to anneal firstand second windows 36 and 38 while the excimer laser is not in use.

Alternatively, windows 36 and 38 are thermally annealed. Thermalannealing is accomplished by heating first and/or second windows in anenvironment such as an inert gas or under vacuum. Although thetemperature to which the windows are heated is dependent on the level ofinduced absorption, a temperature of from about 200 to about 800° C. istypical.

Although preferred embodiments have been depicted and described indetail herein, it will be apparent to those skilled in the relevant artthat various modifications, additions, substitutions and the like can bemade without departing from the spirit of the invention and these aretherefore considered to be within the scope of the invention as definedin the claims which follow.

1. A method of producing a ≧4 kilohertz repetition rate excimer laserhaving a ≧4 kilohertz laser beam and a second laser beam suitable forannealing optical components of the laser, said method comprising:providing a ≧4 kilohertz repetition rate excimer laser for producing afirst ≧4 kilohertz repetition rate laser beam and a second laser beamsuitable for annealing optical components of the laser, said lasercomprised of a laser chamber and a first and a second dischargeelectrode, said laser chamber having a first optical fluoride crystalwindow and a second optical fluoride crystal window with a laser gas forgenerating said first and second laser beams sealed in said chamberbetween said optical fluoride crystal windows; wherein an electricaldischarge from said first electrode supplies energy to said laser gas toproduce a ≧4 kilohertz repetition rate laser beam and an electricaldischarge from said second electrode supplies energy to said laser gasto produce said second laser beam for annealing said optical components,and oscillating said first laser beam whereby the first laser beam exitssaid first optical fluoride crystal window of said chamber and passingthe first laser beam through said second optical fluoride crystal windowof the chamber to provide said ≧4 kilohertz repetition rate excimerlaser beam; and oscillating said second laser beam whereby the secondlaser beam exits said first optical fluoride crystal window of saidchamber and passing the second laser beam through said second opticalfluoride crystal window of the chamber to provide said second annealinglaser beam; wherein the first ≧4 kilohertz repetition laser beam has apulse energy ≧10 ml/pulse; and wherein at least one of said opticalfluoride windows is a barium fluoride window.
 2. The method according toclaim 1 comprising using an argon fluoride excimer laser gas.
 3. Themethod according to claim 1 comprising using a krypton fluoride excimerlaser gas.
 4. The method according to claim 1, wherein said second laserbeam is generated and used to anneal said optical fluoride componentsbefore, during and after the operation of said first laser beam.
 5. Anexcimer laser comprising: A chamber having one or more windowscomprising an optical fluoride; a first source for a first excimer laserbeam within said chamber; and a second source within said chamber for asecond laser beam for annealing the one or more of said comprisingoptical fluoride crystal windows; wherein said first and second laserbeams are generated in said chamber by a first and a second dischargeelectrode, respectively, and said second annealing laser beam can beoperated for annealing said windows before, during or after operation ofsaid first laser beam, and at least one of said windows is a bariumfluoride window.
 6. The excimer laser according to claim 5, wherein thelaser beam has a pulse energy greater than or equal to 10 mJ.
 7. Theexcimer laser according to claim 5, wherein the first laser beam has arepetition rate of greater than or equal to 4 KHz.
 8. The excimer laseraccording to claim 6, wherein the first laser beam has a repetition rateof greater than or equal to 4 KHz.
 9. The excimer laser according toclaim 5 comprising argon fluoride as source gas for the first and secondlaser beams.
 10. The excimer laser according to claim 6 comprising arsonfluoride as source gas for the first and second laser beams.
 11. Theexcimer laser according to claim 7 comprising argon fluoride as sourcegas for the first and second laser beams.
 12. The excimer laseraccording to any one of claim 5 comprising a krypton fluoride source gasfor the first and second laser beams.
 13. The excimer laser according toany one of claim 6 comprising a krypton fluoride source gas for thefirst and second laser beams.
 14. The excimer laser according to any oneof claim 7 comprising a krypton fluoride source gas for the first andsecond laser beams.